This invention relates in general to an improvement in aircraft engine nacelles and, but not by way of limitation, more particularly, to an arrangement for maintaining laminar flow over at least a portion of a nacelle during aircraft flight to reduce drag.
Roughly half of the drag an aircraft experiences in flight is due to skin friction. Great efforts are expended in attempting to reduce drag because of the fuel savings resulting from any such reduction. It has been estimated that about 4% of the drag experienced by an aircraft using gas turbine engines mounted on the aircraft through a wing or fuselage mounted pylon results from freestream flow of air over the engine nacelle.
A large number of nacelle designs have been developed by engineers seeking to reduce nacelle drag. Nacelles have been shaped to provide maximum natural laminar flow, such as the design described by Lahti et al in U.S. Pat. No. 4,799,633. However, these designs must be optimized for either the flow characteristics resulting from cruise flight or the far different characteristics resulting from take-off conditions.
Attempts have been made to blow air out through holes in an aircraft wing to reduce drag, such as is described by Fleischmann in U.S. Pat. No. 2,873,931. Axially directed ridges have been placed on aerodynamic surface to direct air flow in a manner reducing drag, as disclosed by Rethorst in U.S. Pat. No. 3,588,005. Mechanisms within an aircraft wing have been provided to change the airfoil shape during flight to optimize the wing for flight conditions, e.g., cruise, take-off and landing, as described by Readnour et al in U.S. Pat. No. 5,000,399. While these systems often reduce drag somewhat, the improvement has not been sufficient, in view of the weight and complexity of the required apparatus, for any of them to have come into commercial use.
A great variety of structures involving sucking air inwardly through a porous aerodynamic surface have been developed and thereby endeavor to reduce drag by maintaining laminar flow along the aerodynamic surface. Typically, Dannenberg in U.S. Pat. No. 3,128,973 shows wing panels having a porous surface through which air can be drawn into the wing interior. Glaze shows, in U.S. Pat. No. 3,056,432, permeable woven wire material forming longitudinal portions of an aircraft wing skin. Prior art shows longitudinal slots in an aircraft wing through which air can be drawn into the wing interior. Although these systems may beneficially encourage laminar flow over small areas or along longitudinal lines, problems remain with obtaining uniform inward flow over large aerodynamic areas. Also, many of the prior systems for drawing air into a wing or the like present a rough surface or surface discontinuities that will tend to increase drag by causing boundary layer separation. Also, these systems are useful only over large, smooth surfaces, such as aircraft wings, and cannot accommodate turbulence inducing gaps in the surfaces, such as the inherent gaps around movable doors or panels.
Rose et al describe, in U.S. Pat. No. 4,479,150, a boundary layer control system for use in a nacelle that has a layer of honeycomb sound suppression material on the internal surface of the nacelle skin. A porous skin made up of a fine woven mesh is adhesively bonded to a perforated aluminum sheet. The honeycomb sound suppression system is bonded to the opposite side of the aluminum sheet. Suction headers engage the inside surface of the honeycomb, with an impermeable skin between headers. Honeycomb walls are partially cut-away to permit airflow through the honeycomb cell walls and the perforated sheet to reach the headers. While generally effective, this system could be improved, since the woven skin sheet does not provide optimum skin smoothness and the air flow path through the perforations, honeycomb walls and headers is less than ideal.
Thus, there is a continuing need for improvements in laminar flow control over aircraft engine nacelles, that have reduced weight and complexity, provide a smooth exterior skin surface, improve system air flow, provide uniform suction over large areas and can accommodate and protect against the turbulence inducing affects of gaps around nacelle doors and the like.